Design Methodology for Fatigue Evaluation of PVC-O Pipes Using Long-Term Cyclic Internal Pressure Testings

PVC-O molecularly oriented polyvinyl chloride pipe is an improved, higher-performance version of conventional PVC pipe. The improved properties of PVC-O over conventional PVC pipe are due to physical changes in the PVC material caused by realignment of the molecules after extrusion. Molecular orientation increases the pipe’s pressure capacity by raising the tensile strength of pipe materials in the hoop direction; therefore, for the same pressure capacity, the pipe wall thickness is less than that of standard PVC pipes. In actual field conditions, pipelines often experience not only constant pressure but also repeated pressure surges and fluctuations daily. These repeated pressure fluctuations can cause repeated tensile stress in the pipe wall, potentially leading to fatigue failure of the pipe. This paper presents an experimental study to conduct high-pressure long-term cyclic loading (fatigue) tests on 6 in. PC235 PVC-O pipe samples. In this testing, seven pressure ranges (maximum and minimum pressures) will be evaluated, and a minimum of three pipe samples will be tested at each pressure range, with a targeted number of cycles of 2 million. Therefore, the total number of 21 pipe samples will be tested in this testing program. The results of this test will determine the number of cycles at which the PVC-O pipe failed, along with the corresponding pressure range. This supports the development of a fatigue-based design methodology that can predict the number of cycles that a PVC-O pipe can handle at different stress amplitudes. The testing results will be highly beneficial and applicable to the design of the piping system, which often experiences large-magnitude surges. Thus, the testing will predict the cyclic behavior of the PVC-O pipe and determine the number of cycles a PVC-O pipe can withstand before failing, corresponding to a specific stress amplitude.